1. Field of the Invention
This invention relates to a frame synchronizing method and system for a data stream that is modulated using an M.sup.2 code.
2. Description of the Prior Art
There are a variety of conventional digital data modulation systems. One system using an M.sup.2 code is disclosed in U.S. Pat. No. 4,027,335. The M.sup.2 code is based on the miller code and is the so-called DC-free code in which a direct current component can be eliminated after modulation.
The M.sup.2 code is a self-clocking enable code having a minimum inversion interval T.sub.min =T, a detection window width Tw=1/2T, and a maximum inversion interval T.sub.MAX =3T (T.sub.MAX /T.sub.min =3) for a bit cell length T and a code having a range for which the data rate does not become high.
As a result, the recording of video data modulated with the M.sup.2 code onto a tape has generally been performed as part of the digital VTR process. The modulation rule of the Miller code is applied to that of the M.sup.2 code with an improvement for eliminating a DC component of the data so as to overcome a disadvantage in the Miller code. Basically, a "0" of original data is represented by a first level transition of a bit cell, while a "1" of the original data is represented by an intermediate level transition of the bit cell. Further, with respect to a "0" subsequent to a "1" of the original data, the level transition of the bit cell is suppressed. This is the modulation rule of the Miller code. In the modulation rule of the M.sup.2 code, the level transition of the last "1", for example, is suppressed to eliminate a DC component of post-modulation data if an even number of "1's" continue after a "0" of the original data. As a result of such modulation, the above-mentioned various conditions are satisfied.
To establish the correct frame synchronization at the time of digital data reproduction a sync pattern which is clearly distinguished from data must be inserted into the data stream. For instance, in an 8-10 modulation system used in RDAT, it is possible to form a unique sync pattern (which does not occur in a data stream). However, in the above-stated M.sup.2 code, a unique sync pattern is not defined in the encoded data stream. For this reason, a method for inserting a particular (sixteen-bit, for example) sync pattern for frame synchronization between data blocks before encoding has been adopted generally.
As has been mentioned before, since a particular sync pattern is inserted into a data stream, this sync pattern cannot be a unique pattern. Thus, there is a certain probability that the identical pattern may occur in the data stream.
As a result, there is a probability that the above-mentioned data may be detected erroneously as the sync pattern. In short, there is a problem that the synchronization shift and a synchronization error may take place making the establishment of correct synchronization and, as a result, making the correct reproduction of digital data difficult. Also, in a conventional methods, because a particular sync pattern is inserted into data blocks before the M.sup.2 code encoding then modulated collectively by the M.sup.2 code, there is a problem that the sync pattern can be detected only after decoding. Therefore, an improvement on this problem has been desired.